(full disclosure: I am not a video/media person at all - I have only a loose understanding of their job and workflow) Let's say I am authoring a video clip for blu-ray. I have a scene transition that goes to solid black. Let's further say that the tool I am using allows me to insert such "screens" and I define them with hex color values ... so I simply say "fill screen with #000000 for 2 seconds". Which essentially turns into a display resulting in 1920x1080 pixels of that hex code. For 30-60 frames per second. Why aren't events like this huge sources of known-plaintext ? Presumably this could occur in any digital medium - programmatic silence in a digital audio clip, or screens/fillers/pages like the one described above in both satellite and cable TV broadcasts (although the blu-ray example is easier to deal with, since you're not dealing with noise/errors/etc.)